Modulus Demolisher Kit
This includes everything you need except solder and basic tools. If you are newer to modding, I would highly recommend my Nerf Modification guide, which walks you through a stryfe mod, electrical basics, soldering and safety. For the Demolisher, I highly recommend going the MOSFET route, which allows for using light gauge wiring to switch the flywheels on and off. This will let you keep the stock switch in place, since the demolisher is tight inside.
Don't forget that you'll need 2x motors!